Having an aggressive fish can make it difficult to put other fish into your aquarium. Some aggressive fish will compete with others over resources, while others primarily become aggressive to establish territorial dominance. If you think you're ready to add one of more new fish to your existing aquarium and are worried about an aggressive fish in the tank, learning how to select a compatible fish and take the proper precautions can help reduce the risk of tank attacks and bullying.

Steps

Part 1

Choosing the Right Fish

1

Consider the breed. Though you may not realize it, some fish breeds are more prone to being aggressive or semi-aggressive than others. These breeds of fish may require more space through a larger tank, or they may simply need to be kept in isolation.[1] It's important to choose carefully when you select a breed.[2] While there may be exceptions, the following fish are generally considered aggressive or semi-aggressive:

goliath tigerfish

vampire tetra

peacock bass

snakehead fish

piranhas

Arowana

tiger barbs

betta fish

most types of cichlid

2

Factor in the fish's gender. While some breeds are simply more prone to aggression than others, the gender of any given breed may also be a consideration. For example, paradise fish are generally peaceful creatures, but the males are known to display acts of aggression towards other fish (including other paradise fish).[3]

Ask a fish expert (either a pet store employee or a local wildlife biologist) whether your chosen fish breed(s) would cohabitate better if they were all male, all female, or a mix.

3

Evaluate other competitive factors. Depending on the types of fish you're combining, an otherwise non-aggressive fish may become aggressive over time. Two of the biggest factors are space and resources, though of course you'll have to consider the behavior of each individual species as well.[4]

How big will each fish get? Will they have enough room, or will they begin to compete for space and resources?

Are any of the types of fish you're considering combining known to eat other fish? If so, this will almost certainly create a hostile situation no matter what precautions you take.

4

Decide how many fish to get. Once you've decided to combine fish into the same aquarium, you'll also have to consider how many fish you want to add. You might logically think that fewer fish will mean a reduced risk of aggression, but that isn't always necessarily the case.[5]

Some nonterritorial fish breeds, like tiger barbs, are known to become aggressive when kept in too small a school.

In the case of tiger barbs, adding more fish of this breed actually reduces the risk of them becoming aggressive with one another.

Talk to a fish expert before deciding on how many fish to add.

Part 2

Adding New Fish to Your Aquarium

1

Add lots of hiding places to your tank. The best way to reduce the risk of aggression is by reducing the risk of confrontation. Most fish fight over territory, so by providing a lot of micro-territories you can ensure that each fish has its own distinguished space.[6]

Hiding places serve two purposes: they offer your fish a sanctuary to hide in, and they can serve as their own micro-territories for each fish to lay its own claim to.

Choose fish accessories with hollowed-out sections, as these make great hiding places.

Check your local pet store for any accessories that have tunnels or other hollow spaces.

You can also add rocks with hollow "caves," if your tank has sufficient room.

2

Distract the existing aggressive fish. When the time comes for you to add the new fish to your existing tank, you'll want to distract the existing aggressive fish as much as possible. The easiest way to do this is by using the so-called "mirror trick."[7]

Stand a mirror outside the tank where the aggressive fish can see it. It should be close to the tank, so either stand it up near the glass or find a way to press it up against the tank.

The aggressive fish will be distracted by its reflection. In the meantime, your new fish can get acclimated and establish its own territory.

Leave the mirror there until the new fish has settled in.

3

Introduce new fish simultaneously. If you're going to introduce more than one new fish into an established aggressive tank, you may want to add them all at once. This allows each fish to establish its own territory simultaneously, which will cause less conflict than if several aggressive fish already had their own claimed territories.[8]

You should still distract any existing aggressive fish with a mirror prior to adding new fish, even if the new fish are added at the same time.

Make sure there are a sufficient number of hiding places for each fish before you add them to the existing tank.

4

Give your fish more food and multiple sources of food. Aggressive fish don't just fight for social order; many also fight over resources like food. When you introduce new fish into an aggressive aquarium, you'll want to ensure that you give the fish more food than usual. You may also want to consider providing more than one source of food.[9]

Consider feeding your fish simultaneously from both ends of the tank to allow feeding across the aquarium.

Part 3

Getting Your Fish to Cohabitate

1

Let them fight it out. Though it may sound counterintuitive, sometimes it's best to let new fish fight it out (so long as none of the fish appear to be trying to kill the others). Fish establish an order of dominance that puts the most aggressive/dominant fish at the top of the hierarchy. [10]

Some fish will continue to fight, in which case further measures will be necessary. However, many fish will settle into their newly-adopted roles once they hash things out.

Keep an eye on your fish. If the most dominant one continues to bully and harass the others, you'll need to change up their environment to correct the problem.

2

Change the environment. If your fish have been fighting it out and it seems like one is simply bullying the other(s), you may need to change up the aquarium's layout. Changing or rearranging the environment can help "reset" each fish's established territories.[11]

Try rearranging the accessories inside your tank. Take out one or two old ones and add as many new ones as you can, or simply change the location of existing accessories in the tank.

If rearranging the tank doesn't work, you may need to try moving all of your fish into a new, larger tank and providing them with brand new accessories/hiding places.

3

Separate highly aggressive fish. If changing the environment hasn't worked and your fish won't stop fighting no matter what you do, you may need to bite the bullet and separate them into two or more different tanks. If one or more highly aggressive fish relentlessly attack another fish, that fish could eventually suffer from infections or further injuries, resulting in death.[12]

Most aquarium enthusiasts choose to remove the fish that is getting attacked the most. However, you may choose to remove the aggressive fish instead.

If there are other fish in the tank, be aware that they may try to establish a new order of dominance once a fish (either the aggressive one or the victim) is removed.

Another option is to purchase a net breeder from your local pet store. This device hangs inside the tank and allows you to isolate the aggressive fish within its own tank while the other fish establish their own territories.

If using a net breeder, keep the most aggressive fish isolated for about one week. This should be enough time for the order of dominance to adjust or reset accordingly.

Continue to monitor the fish to ensure that they find some type of stable order and eventually cohabitate.

Community Q&A

Clown plecos make great tank mates for bettas, though they need a slightly larger tank. Pygmy corydoras, ember tetras, and harlequin rasboras can also make good tank mates with bettas. White cloud mountain minnows make excellent tank companions, but they prefer a water temperature between 60 and 75 degrees Fahrenheit (15.5 to 24 degrees Celsius), while bettas prefer a temperature between 75 and 80 degrees Fahrenheit (24 to 26.5 degrees Celsius). If choosing white clouds, keep the tank consistently around 75 degrees Fahrenheit (24 degrees Celsius).